The invention relates to a control system and method for controlling a power shift transmission of a utility vehicle, such as an agricultural tractor.
It is known to provide an agricultural tractor with a power shift transmission which can be shifted under load. The driveline for such a vehicle typically includes an engine, a power shift transmission controlled by actuating mechanism, a clutch, a synchronized transmission that can be shifted by a shift lever, and driven vehicle wheels.
Commercially available agricultural tractors, such as John Deere tractors in the 6000 series, have drivelines which are driven by an internal combustion engine and which include a shifted multi-speed transmission, a reversing unit, a clutch, an optional creeper transmission, a shifted range transmission, a rear axle differential and the driven rear wheels. The shifted multi-speed transmission is configured as a power shift transmission, (also called a partial power shift transmission) and includes planetary gear sets with clutches and brakes. Such a transmission may be shifted under load. Gear shift control signals are transmitted from a gearshift lever to a control unit which controls an actuating mechanism which causes the power shift transmission to shift. The range shift transmission is a fully synchronized shifted transmission whose ranges can be actuated by a range shift lever. A direction control lever is used for the shifting of the reversing unit between forward and reverse directions. This known driveline has a smooth succession of gear ratios and makes possible shifting under load within a selected range of gear ratios. The operator must manipulate two gearshift levers in order to utilize the smooth succession of gear ratios and adjust the gear ratio of the power shift transmission during a shift of the range transmission for many gear ratio combinations.